An ink-jet printer includes a pen in which small droplets of ink are formed and ejected toward a printing medium. Such pens include a printhead having an orifice member or plate that has several very small orifices through which the ink droplets are ejected. Adjacent to the orifices are ink chambers, where ink resides prior to ejection through the orifice. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir portion of the pen.
Ejection of an ink droplet through an orifice may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. This thermal process causes ink within the chamber to superheat and form a vapor bubble. Formation of thermal ink-jet vapor bubble is known as nucleation. The rapid expansion of the bubble forces a drop of ink through the orifice. This process is called "firing." The ink in the chamber is typically heated with a resistor that is aligned with the orifice.
Once the ink is ejected, the ink chamber is refilled by capillary force with ink from the ink channel, thus readying the system for firing another droplet.
As ink rushes in to refill an empty chamber, the inertia of the moving ink causes some of the ink to bulge out of the orifice. Because ink within the pen is generally kept at a slightly positive back pressure (that is, a pressure slightly lower than ambient), the bulging portion of the ink immediately recoils into the ink chamber. This reciprocating motion diminishes over a few cycles and eventually stops or damps out.
If a droplet is fired when the ink is bulging out of the orifice, the ejected droplet will be large and dumbbell shaped, and slow moving. Conversely, if the droplet is ejected when ink is recoiling from the orifice, the ejected droplet will be small and spear shaped, and move undesirably fast. Between these two extremes, as the chamber ink motion damps out, well-formed drops are produced for optimum print quality. Thus, print speed (that is, the rate at which droplets are ejected) must be sufficiently slow to allow the motion of the chamber to damp out between each droplet firing. The time period required for the ink motion to damp sufficiently may be referred to as the damping interval.
To lessen the print speed reduction attributable to the damping interval, ink chamber and ink channel geometry may be optimized. Specifically, ink channel length and area may be constructed to restrict the ink flow rate into and out of the chamber, thereby to reduce the reciprocating motion of chamber refill ink (hence, lessen the damping interval). In the past, ink channels have been relatively long with respect to the area, hence the length of the channel is a necessarily important consideration in optimizing damping characteristics of the channel.
Prior ink-jet printheads are also susceptible to ink "blowback" during droplet ejection. Blowback results when some ink in the chamber is forced back into the adjacent part of the channel upon firing. Blowback occurs because the ink in the chamber is not separated from the ink in the channel. Accordingly, upon firing, a large portion of ink affected by the expanding bubble within the chamber is blown back into the channel instead of out the orifice. Blowback increases the amount of energy necessary for ejection of droplets from the chamber ("turn on energy" or TOE) because only a portion of the entire volume of ink in the chamber is actually ejected. A higher TOE results in excessive printhead heating.
Excessive printhead heating also generates bubbles from air dissolved in the ink and causes prenucleation of the ink vapor bubble. Air bubbles within the ink and prenucleation of the vapor bubble result in a poor ink droplet formation and, thus, poor print quality.
Components of the printhead in the vicinity of the vapor bubble are susceptible to damage from cavitation as the vapor bubble collapses between firing intervals. Particularly susceptible to damage from cavitation is the resistor. A thin protective passivation layer is typically applied over the resistor. The application of a passivation layer over the resistor, however, increases the TOE necessary for ejecting droplets. Put another way, the trade-off in efforts to reduce TOE by thinning the passivation layer is reduction in the protection against cavitation damage of the resistor.
The present invention provides a printhead construction that situates an ink inlet contiguous with the camber and immediately adjacent to the resistor in each chamber of the printhead. The ink inlet defines the path through which ink passes from the ink channel and into the chamber. The inlet is sized and located so that as the vapor bubble expands to fire ink from a chamber, the bubble simultaneously moves into the inlet to separate the ink within the chamber from the ink within the channel, thereby occluding any liquid pathway between the chamber and channel as ink is ejected from the orifice. This occlusion of a liquid pathway between the chamber and the channel during firing minimizes blowback. In a sense, therefore, the vapor bubble acts as a valve as it expands to occlude the inlet, hence temporarily stopping ink flow out of the chamber (blowback) during the firing process.
The blowback resistance attributable to this bubble valving raises the system thermal efficiency, lowering TOE. A lower TOE reduces printhead heating. Reducing printhead heating helps maintain a steady operating temperature, which provides uniform print quality.
As another aspect of this invention, the flow length of the inlet, which is the distance between the ink chamber and ink channel through the inlet, is relatively short. Moreover, the volume of ink in the ink channel is substantially greater than that of the ink inlet. As a result, an amount of damping of the ink flow into the chamber may be optimized by consideration of only the area of the inlet.
As another aspect of the present invention, the inlet is arranged so that the ink flows into the chamber along a flow path that, in preferred embodiments, is substantially parallel to the central axis of the orifice, along which axis ink droplets are expelled from the chamber. As a result, the flow of the ink to refill the chamber, which flow commences as the vapor bubble begins to collapse, provides momentum for lifting the collapsing bubble from the resistor so that the eventual collapse point of the bubble is displaced from the resistor, thereby minimizing the damaging effects of cavitation on the resistor that would otherwise occur were the vapor bubble to collapse substantially on the surface of the resistor.